1. Field of the Invention
The present invention generally relates to D/A (Digital-to-Analog) converters, and more particularly to a D/A converter including an offset-controllable amplifier used as an output buffer.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a conventional D/A converter fabricated with a resistor network. A resistor network is made up of resistors R.sub.0 -R.sub.k-1 connected in series. The resistors have the same resistance as each other. A ground potential (E.sub.0 =0 V) is applied to one end of the resistor network, and a power supply voltage V.sub.R (=E.sub.k) is applied to the another end of the resistor network. Analog switches S.sub.0 -S.sub.k are connected to the resistor network. More particularly, first terminals of the analog switches S.sub.0 -S.sub.k are connected to respective nodes of the resistor network, and second terminals of the switches are connected to each other and to the non-inverting input terminal of an operational amplifier OP. One of the (k +1) analog switches S.sub.0 -S.sub.k is selected on the basis of a digital input signal value, and the potential at the corresponding node is applied to the non-inverting input terminal of the operational amplifier OP.
The inverting input terminal of the
operational amplifier OP, which functions as a unit-gain amplifier, is connected to the output terminal of the operational amplifier OP. The operational amplifier outputs an analog output voltage V.sub.OUT corresponding to the potential of the selected node.
The correspondence between the selecting operation on the analog switches S.sub.0 -S.sub.k and the analog output signal V.sub.OUT can be expressed by the following equation when only one S.sub.X (X is one of 0-k) of the (k +1) switches is ON and the other switches are OFF: EQU V.sub.OUT =E.sub.X =(X/k).multidot.V.sub.R.
It can be seen from the above equation that (k+1) analog voltages (E.sub.0, E.sub.1, E.sub.2, . . . , E.sub.k-2, E.sub.k-1, E.sub.k) having a step size equal to (1/k)V.sub.R are obtained by turning ON only one of the (k+1) switches.
The D/A converter shown in FIG. 1 functions as an n-bit D/A converter by controlling the (k+1) analog switches S.sub.0 -S.sub.k by means of decoded signals of a digital input signal (an n-bit digital input signal is decoded into 2.sup.n decoded signals). For example, when all bits of the digital input signal are equal to zero, the decoded signals turn only the switch S.sub.0 ON, and the analog output voltage V.sub.OUT is equal to 0 V (=E.sub.0). When all bits of the digital input signal are equal to one, the decoded signals turn only the switch S.sub.k ON, and the analog output signal V.sub.OUT is equal to V.sub.R (=E.sub.k). When the bits of the digital input signal have values other than all zeros and all ones, one of the switches S.sub.1 -S.sub.k-1 is turned ON in response to the decoded signals, and a corresponding one of the output voltages E.sub.1, E.sub.2, . . . , E.sub.k-1 is output as the analog output voltage V.sub.OUT.
The number of analog switches needed is equal to the number of decoded signals. That is, 2.sup.n decoded signals are needed to perform the D/A converting operation on the n-bit digital input signal. Hence, 2.sup.n analog switches are needed. Further, a minimum number of resistors needed is equal to 2.sup.n -1.
However, the above-mentioned conventional D/A converter has a disadvantage in that the number of required parts, such as analog switches and resistors exponentially increases as the number of bits of the digital input signal increases. For example, when the number of bits to be handled is increased from 8 to 10, the number of analog switches must be increased from 2.sup.8 (=256) to 2.sup.10 (=2.sup.8 .multidot.2.sup.2). That is, the number of analog switches necessary to process 10 bits increases by four times the number of analog switches necessary to process 8 bits. Further, in the above case, the number of resistors must be increased from 2.sup.8 -1 (=255) to 2.sup.10 -1 (=2.sup.8 .multidot.2.sup.2 -1=1023). That is, the number of resistors necessary to process 10 bits increases by approximately four times the number of resistors necessary to process 8 bits.
Compared to the 8-bit D/A converter fabricated on an IC chip, a very large area on the chip is needed to form the 10-bit D/A converter. Exactly, the on-chip D/A converter needs a chip area approximately equal to four times the area for 2.sup.8 switches and a chip area approximately equal to four the area for 2.sup.8 -1 resistors.